Hello, Dakota Bill.. I'm trying to help you in your search about the shotgun.
The commercial name is "Robust" it was made by the "Manufacture d'armes et cycles de St Etienne" (ManuFrance since the sixtie's). These manufacture was a commercial one, all the others like MAS, MAT or MAC are government manufacturers, they never done any shotguns for civil-hunting use.
The serial number of your shotgun is under the "opening-key", move the key, you can see the number whith one "prefix": 221 or 222-
221: your gun come whith a "normal" sling
222: it come whith a "retractable" automatic sling. The sling -house is in the butt. The production stop in 1982, cause the closing of the manufacture. During the 50 last years of production barrels come whit hard chrome plating. A look under the barrel "table", you can find these stamps: 65 or 70 mm( 2'1/2 or 2'3/4) for the case length. The number 28 is,may be, for the "28 grammes de plomb", normal charge of led in a 16 gauge ammo. 170, may be 17.0 is the diameter of the bore, three diam.: 16.8mm, 16.9mm or 17.1mm was the most common. "Charge d'epreuve " et "charge normale", normally you can find some symbols after, they give you the pressure of the proof charges used for the testing of the gun.
S.G.D.G : "Sans Garantie Du Gouvernement" ( Don't be afraid ,it was a common stamping on all products made in france in private manufactures...) These shotguns was and are very popular in France, they are very reliable, the name "robust", rugged in english is perfect for these gun. Some of the early series became collectible shotguns in France now. In both chambering 16 or 12 gauge..